Large conductance Ca2+ -and voltage-dependent K+ channels (termed BK channels) are ubiquitously distributed ion channels that couple changes in submembrane Ca2+ to regulation of cellular electrical excitability. This coupling of BK channel activation to Ca2+ influx plays a negative feedback role to terminate Ca2+ influx during action potentials. This is critical to processes as diverse as the termination of transmitter release at synapses, regulation of hormone secretion from neuroendocrine cells, and the frequency tuning of hair cells in the cochlea. This proposal seeks to define the key properties of BK channels underlying their physiological roles. Furthermore, the proposal will evaluate the hypotheses that the BK channel may play a key role in the regulation of electrical excitability and secretion, in part, because of a selective association with particular subtypes of Ca2+ channels. To address these issues, patch-clamp recording methodologies, confocal imaging of immunohistochemically localized ion channels, and carbon fiber electrode (CFE) recordings of quantal secretory signals from individual rat adrenal chromaffin cells will be utilized. The proposal begins with an examination of how BK channel activation may be critically dependent on selective coupling to particular Ca2+ channels. It then proceeds to the physiological roles played by BK channels and the implications that coupling of BK channels to Ca2+ channels may have both for regulation of cellular excitability and secretion. The principles developed here are likely to be of significance not only for neuroendocrine cells, but for all excitable cells where BK channel activation is dependent on the high concentrations of Ca2+ only found in the vicinity of open Ca2+ channels. Abnormal function on localization of BK channels will impact significantly on normal excitable cell function. Thus, understanding the role of BK channels both in regulation of transmitter release and in regulation of neuronal excitability is a critical element in being able to identify the key alterations that occur in association with pathologies of synaptic transmission, secretion, or excitability.